As it is known, gas turbine engines, in particular their compressors, are affected by fouling and therefore need to be cleaned repeatedly during their lifetime.
A common way to clean a gas turbine engine consists in interrupting its normal operation and washing it, without disassembling the engine. This is the so-called “off-line” washing and is carried out by means of a liquid detergent. After treatment with the liquid detergent, rinsing is often necessary. Off-line washing is very effective; anyway, it implies interrupting normal operation and therefore increases the downtime of the machine and of the plant including the machine.
It is also known, even if less common, to wash a gas turbine engine during operation, i.e. when the engine generates work. This is the so-called “on-line” washing and consists in adding a liquid detergent to the gas flowing in the compressor. In this case, the quantity of liquid detergent added to the gas is small (more precisely the liquid-to-gas ratio is maintained low) and the pressure of the ejected liquid detergent is low in order to avoid:                disturbing the operation of the compressor and/or the turbine and/or the combustor (for example the combustion may extinguish due to the liquid detergent),        disturbing the fluid flow inside the compressor,        damaging the components of the compressor (for example liquid detergent droplets, if any, may hit against e.g. the rotating blades of the compressor).        
It is to be noted that liquid detergents use for “off-line” washing are usually different from liquid detergents used for “on-line” washing.
Known on-line washing methods are much less effective then known off-line washing methods, even if they have the advantage of not affecting the downtime of the machine and of the plant including the machine.
It's also known from the document “Online Water Wash Tests of GE J85-13” by Elisabet Syverud and Lars E. Bakken to wash a gas turbine injecting water, with a water-to-air ratio ranging from 0.4% to 3% by mass, while the machine is operating.